This study is designed to develop an in vitro potency assay to replace the guinea pig death test portion of official release test for the tetanus component in adsorbed toxoid vaccines. An ELISA method patterned after that reported in the literature, has been successfully adopted by the Laboratory of Standards and Testing. We continue to evaluate sera that has been tested in animals in the normal course of incoming product evaluation. The method of analysis has included statistical comparison between actual miciogram of tetanus antibody in different preparations and the determination of the relative potency of a pool of immunized animals to a quantitated reference. We are currently attempting to locate a source of purified toxin or toxoid suitable for the ELISA procedure that could be used in a collaborative study with other laboratories.